(a) Color calibration and correction—Color calibration is a known function in color printers. Its objective is to provide consistency of color within an image, and among all images printed by a given printer, and from printer to printer.
Thus a proper color-calibration algorithm (CCA) compensates for printer deviations in such a way that the same nominal colorant values—i.e. quantities of cyan (C), magenta (M), and yellow (Y) ink, and black (K) if present—produce the same output from any printer which undergoes the calibration. It is helpful to consider a CCA as influencing 366, 368 (FIG. 15) a color-correction stage 365, or the breakpoints 367 (i.e. the threshold values) used in rendition, or both.
Conventional color correction, sometimes referred to as a “transfer function”, is a one-dimensional mapping (FIG. 16) for each colorant 381–84 respectively. In eight-bit data processing for incremental-printing systems, ordinarily the color-correction mapping is from eight bits of nominal colorant (C, M, or Y, or K if present) to eight bits of printer-specific colorant.
Various ways of forming a color-correction mapping are known. In some products of the Hewlett Packard Company, such mappings have been configured with the specific aim of preserving the linearity of the colorants C, M and Y—and again K if present.
Experiments have shown, however, that linearity of colorants, while providing an adequate solution for certain kinds of color variations such as those caused by drop-weight fluctuation, nevertheless has distinct limitations. These limitations are particularly troublesome for inkjet printing. First, when primary colorants exhibit a hue shift—such as often caused, with certain media, by high humidity—the primary-linearity technique helps only very little.
Second, this technique fails to ensure a critical condition which is a hallmark of highest-quality printing systems: gray neutrality, or in other words absence of chroma, in nominally gray image features printed as combinations of the primary chromatic colorants C, M and Y.
(b) Composite or process black and gray—It is well known that combinations of these three subtractive chromatic primaries produce a close approximation to black, often called “process black”. In the incremental-printing industry, composite/process black or gray when occurring outside highlight regions is usually replaced by actual black ink when available.
The object of such replacement is to reduce both ink usage and the volume of liquid deposited on the printing medium—and also to circumvent possible problems due to inaccuracy of the process-black approximation to actual black. Not all incremental-printing devices, however, have true-black ink cartridges. Therefore, in some such devices, composite black is the only way to achieve any black, and in such systems the accuracy of the process-black approximation assumes greater importance.
In incremental printing an important use of process black, or more precisely process gray, is for the benefit of its mechanical capability to spread or distribute, over a broader image area, colorant that appears neutral to the eye (see the Perumal document mentioned earlier). In this case the chromatic primaries are not overprinted but rather are adjacent—or even scattered rather widely—so that the overall impression of the visually integrated dots is of a smoother or silkier texture, though still one of a very light gray. Therefore, in this rather sophisticated case, process gray is important even if actual black ink is available.
(c) Inaccuracy—When used with less finesse, however, process black—particularly in incremental printing—tends more toward being merely inaccurate. Discerning observers detect some faint hue, some chromatic component, in image areas that are nominally gray.
This chromatic component arises from imperfectly balanced proportions of the three subtractive primary colorants. The idea of “perfectly balanced proportions” unfortunately is ephemeral, because ideal proportions actually vary with the chemical and calorimetric characteristics of the specific colorants employed.
Ideal proportions also vary with the electromechanical characteristics of the printheads used to apply the colorants to the printing medium. All these factors typically vary from batch to batch of colorants and heads.
Furthermore these characteristics interact in confounding ways with characteristics of the printing medium, and of the sequence and even the timing of colorant deposition—and these characteristics interact with each other as well. The difficulty does not stop there, as ambient conditions including temperature and humidity also interact with the foregoing factors to prevent any stable, single set of simple weight or volume proportions from being usable over the life of a printing device.
The hue that appears in nominally gray regions, being uncontrolled, is most typically irrelevant to the subject matter of the particular image features. Esthetically, therefore, it can often be quite jarring.
In perhaps more-technical terms, what is being perceived is nonzero chroma. Colors that should be on the central black-white axis of a theoretical perceptual color space are instead reproduced slightly off-axis in one or another direction within that space.
Such effects are least conspicuous in shadow and highlight regions, where chroma is very difficult to detect visually anyway. They are most obtrusive in midtone regions, where chroma and hue are dominant characteristics of human perception.
In incremental printing it is relatively rare for artists to specify any particular inking effects for particular regions of an image. At least when using low-end systems it is rather difficult even to gain access to controls for such effects.
Instead the admixtures of physical colorants are simply left to the machine, without differentiation as to the specific subject matter. Therefore most incremental printing is particularly vulnerable to the adverse effects of process black used unskillfully.
(d) Earlier correction of process-black chroma—It is accordingly of particular importance that when process black is used it be accurately black—that is to say accurately neutral, nonchromatic. As noted above, however, the configuring of color-correction mappings to preserve linearity in primary-colorant ramps fails to provide this characteristic.
Some earlier products of the Hewlett Packard Company use a color-correction mapping which is embodied in a calibration lookup table (see Bockman, mentioned above). The table is formulated in the laboratory, most typically before a production line opens for a particular product. In field operation, such a table is then read by a system that is open-loop as to chroma—i.e., a system with no feedback of field-measured gray-neutrality information to the color-correction stage.
Other such products do print, measure and respond to color test patterns in the field, but not with respect to actual neutrality of nominally neutral patches made with composite black/gray. More specifically, it is known to canvass or assay generally throughout an entire device gamut, approximating much of a color space. Although some colors thus sampled and measured may be near the neutral axis, this technique essentially approaches neutrality on an incidental basis, and the actual neutrality of grays achieved is correspondingly catch-as-catch-can.
It is also known—essentially as an opposite extreme—to step through colorimetric measurement of individual-colorant ramps. This technique seeks to approach the overall calibration as a matter of linearity of such ramps, as suggested earlier. Without more, this method as well yields inconsistent grays.
(e) Composite secondaries, and inaccuracy—Colors that are well known as additive primaries in video work (where all effects arise as colored lights) occur instead as “secondaries” in printing (where all effects arise from subtractive primary colorants). In printing therefore red, green and blue are secondary colors, usually generated by adjacent or superposed yellow plus magenta, yellow plus cyan, and cyan plus magenta, respectively.
The accuracy of each secondary accordingly depends upon accuracy of the proportions of the subtractive primaries used. For instance the accuracy of red, formed from yellow and magenta, depends upon the accuracy of proportions of the yellow and magenta colorants used.
Here too, as in the foregoing process-black discussion, the definition of “accuracy of proportions” is a very elusive concept because optimum proportions really depend upon a complex of attributes, including those of the colorants, colorant-application devices, printing medium, deposition sequence and timing, temperature and humidity. Nevertheless, just as there is a clear definition of what is meant by “gray neutrality”, it is possible to fashion clear (if spectrally complicated) definitions of what is meant by “red”, “green” and “blue”.
When these colors are not produced accurately, the resulting esthetic impression can be even more troublesome than slight chroma within regions that are nominally gray. This is so for two reasons.
First, only limited sorts of objects in color photos depend for their realism upon total absence of chroma. Second, inaccuracies in the color secondaries manifest themselves as hue shifts, to which observers typically respond by saying that the colors are “off”.
Results can be especially conspicuous in flesh tones that have a strikingly unnatural cast, or in other objects of well-known but inaccurately rendered hue that observers may describe as “wrong”. Earlier efforts to deal with the problem of inaccurate secondaries have suffered either from complete absence of secondary-accuracy feedback information or—in systems that rely on field spectral measurements using wideband sensing—at least from absence of reliable hue references for those colors.
(f) Conclusion—Chroma appearing in nominally gray regions, and secondary-color hue errors, have continued to impede achievement of uniformly excellent inkjet printing. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.